Known differential pulse code modulation (DPCM) communications systems for speech coding, for example, are made adaptive in step sizes from sample to sample in order to compensate for two types of quantization error: slope overload distortion and granular noise. These types of quantization error reflect situations where the quantizer step size is either too small or too large relative to the amplitude change occurring between samples. The solution has been to increase step size during overloads and to decrease it during granularity. The practice has been to increase and decrease the step size by preselected multipliers, thus expanding and contracting the dynamic quantization range without changing the number of encoding bits per step or the uniform spacing between them. Where step size is adjusted after each sample, the quantization is said to be instantaneous. Where step size is adjusted at longer than sample intervals, the quantization is said to be noninstantaneous or syllabic.
A DPCM coding system in which the step size is changed in accordance with the amplitude of the latest sample, or of an average of a block of samples, is said to be adaptive and becomes known as an adaptive DPCM or ADPCM coding system. Such a system is described by P. Cummiskey, N. S. Jayant and J. L. Flanagan in the paper "Adaptive Quantization in Differential PCM Coding of Speech" (Bell System Technical Journal, Vol. 52, No. 7, pp. 1105-1118, September 1973).
FIG. 1 of this paper constitutes a block diagram of an ADPCM system on which the present invention is an improvement. The analog signal at the input to the system is first sampled at a constant rate, e.g., at the Nyquist rate; the samples are next quantized; and then the quantization levels are converted into a binary code for transmission to a remote receiver. The samples are also sent through an integrator loop to reconstruct the original signal. In selecting the first sample, the difference between each reconstructed previous signal, necessarily a less than perfect replica of the original signal due to the quantization, and the next signal sample is continuously applied to the quantizer whose output becomes the transmitted signal. According to the quantization level or slot occupied by the previous difference signal, the quantization levels are expanded or contracted uniformly on an empirical basis before the next difference sample is encoded. At a decoder the reverse of the encoding procedure is accomplished to reconstruct the original analog signal.
The principal advantage of the ADPCM coding system is the minimization of the number of bits needed to transmit the encoded signal as compared to the number of bits required, for example, for conventional eight-bit PCM. The example in the cited paper uses three bits for each difference sample. The number of bits per sample is herein generalized as R-bits. For the three-bit coding there are eight possible levels arranged symmetrically about a reference level. Corresponding to each level a multiplier is assigned to expand the dynamic range of the system when the previous signal is quantized on an outer level and to contract such range when the previous signal is quantized on an inner level.
PCM coders have a property of interest to the present disclosure known as embedded coding. A digital code that admits simple bit dropping and reinsertion is called an embedded code. An eight-bit PCM word can be truncated, for example, by dropping less significant bits to accommodate a slower speed channel and deliver a coarser, but recognizable, signal to a receiver. DPCM, on the other hand, is not an embedded code because dropping bits generates errors in the feedback path around the quantizer in both transmitting and receiving terminals. An embedded code is useful where a variable rate channel must be accommodated. Reducing the number of bits per sample reduces signal quality at a lower signaling rate per word. Increasing the number of bits per sample increases signal quality but either reduces the overall signaling rate or requires a higher speed channel. p It is an object of this invention to provide a variable rate ADPCM transmission system.
It is another object of this invention to provide explicit reconstruction or quantization noise coding in an ADPCM transmission system.